1. Field of the Invention
The present invention relates to a method and an apparatus for forming a thin film utilizing microwave plasma enhanced CVD, and more particularly to a method and an apparatus utilizing the microwave plasma enhanced CVD which is capable of forming a thin film of a large size with uniformity. The present invention further relates to a gas feeding, method and apparatus suitable for the microwave plasma enhanced CVD.
2. Description of the Related Art
In recent years, a liquid crystal display unit having a large display area has been developed and come to be used widely. Such a liquid crystal display unit comprises a panel in which a number of thin film transistors each comprising an amorphous silicon thin film are arranged in a matrix form.
FIG. 6 illustrates, in an enlarged scale, a conventional thin-film transistor as disclosed, for example, in Japanese Unexamined Patent Publication (KOKAI) No. 58-114453. The thin-film transistor comprises a gate electrode 11 formed in a desired position on a transparent glass 10 as a substrate, a gate insulation film 12 formed so as to cover the gate electrode 11, an amorphous silicon i layer 13 formed on the gate insulation film 12, a metal 14 for a Schottky barrier formed in a part of the periphery of the gate insulation film 12 and amorphous silicon i layer 13, a source electrode 15, a drain electrode 16 which covers the periphery of the gate insulation film 12, amorphous silicon i layer 13 and metal 14 and has an opening formed on the entire outside thereof, a passivation film 17 formed on the opening, and a flag film 18 formed on the passivation film 17.
The amorphous silicon i layer 13 and the gate insulation film 12 are formed by using a conventional plasma CVD method and apparatus. According to this method, the substrate is heated to a temperature as high as 250.degree.-350.degree. C. for the desired film deposition. For this reason, the materials of the substrate or other underlayers are limited. In addition, there is a problem of diffusion of impurities. The conventional method involves a further problem that the decomposition or utilization efficiency of a material gas such as silane (SiH.sub.4) is rather low and the yield of the film formation stands low because of reaction products generated in a large amount during the film formation. Besides, it is also a serious problem that due to a high-temperature reaction, a considerable number of comparatively weak Si--H couplings are included in the formed film, which causes a dangling bond defect. The dangling bond has no connectors and forms a trap level of electrons to adversely affect film or transistor characteristics. Consequently, it is difficult to obtain a thin film transistor of which characteristics can be stable and hardly changeable with the passage of time.
To solve the problems involved in the conventional plasma-assisted CVD, there has been proposed an electron cyclotron resonance plasma enhanced CVD method which utilizes electron cyclotron resonance caused by the interaction between a microwave and a magnetic field. According to this method, a film can be formed at a comparatively low temperature as compared with a conventional plasma-assisted CVD method utilizing a chemical reaction at a high temperature. This, therefore, can eliminate the limitation in the materials of the substrate and the underlayers.
By way of example, Japanese Unexamined Patent Publications No. 62-71276 and No. 63-115328 have disclosed a microwave plasma enhanced CVD apparatus in which a magnetic field is generated by an electromagnetic coil in a plasma generating chamber provided separately from a film forming chamber for forming a thin film and a microwave is led into the plasma generating chamber through a magnetron to resonantly absorb electromagnetic energy into electrons in a static magnetic field and to generate a plasma in the plasma generating chamber. The thin film is formed on a substrate depending on the interaction between a gas to be fed into the film forming chamber and the plasma to be fed from the plasma generating chamber into the film forming chamber.
More specifically, in the microwave plasma enhanced CVD apparatus, the plasma which is generated in the plasma generating chamber, provided separately from the film forming chamber, is diffused through a small opening provided in a partition between the film forming chamber and the plasma generating chamber so as to be utilized as a radiation plasma. As a result, therefore, there is caused another problem that the area of the plasma which comes into contact with the substrate having a thin film is restricted and cannot be increased.
On the other hand, a gas for the desired thin film is fed into the film forming chamber through a nozzle or ring-shaped gas spouting port so as to be directed toward the radiation plasma which is diffused through the opening. FIG. 7 shows one form of a conventional microwave plasma, enhanced CVD apparatus, particularly showing a gas feeding configuration thereof as disclosed, for example, in Japanese Unexamined Patent Publication No. 63-43324. In FIG. 7, 3a designates a plasma generating chamber; 4 a magnetic field applying means; 5 a microwave introducing guide; 6a a feeding tube for plasma gas ; 6b a feeding tube for a gas for the thin film; and 7a a film forming chamber. The operation of the apparatus is similar to that mentioned above in connection with publication No. 62-71276 and No. 63-115328.
In general, when the film formation is made in a film forming chamber of a small volume and under a low pressure, for example, in an atmosphere of 0.5 mTorr or lower, the flow (arrow B in FIG. 7) of the gas for the thin film which is introduced through the tube 6b is, in many cases, a molecular flow. Therefore, the tube 6b may have a configuration of a nozzle or a ring gas jetting port to provide a uniform gas flow distribution.
FIGS. 8 (a) and 8 (b) show the results of an experiment conducted by the inventors of the present invention on the characteristics of the thin film formed when a gas for the thin film is introduced through the nozzle disposed in parallel with a substrate. The results show that the gas for the thin film is diffused towards the center of the substrate and has a distribution of deposition rate or film thickness as shown in FIG. 8 (a). There is also a distribution of resistivity of the film formed as can be seen from FIG. 8 (b). Thus, although the gas is diffused along with the plasma, a film thickness distribution and a film deposition rate are gradually decreased from an end of the substrate where the gas is fed. Consequently, it is difficult to form the thin film having uniform properties over the entire substrate. By this reason, the thin film having a desired uniformity cannot have a size larger than .phi.100 mm or so.
Thus, it is quite difficult to obtain a uniform thin film of a desired large size by a conventional microwave plasma enhanced CVD apparatus.